MONDAY, Aug. 23 (HealthDay News) -- When you buy a bottled tea, you may not be getting the health boost you expect.

A new study finds that these increasingly popular beverages may contain far lower levels of antioxidants called polyphenols than green or black tea that you brew at home. In fact, some commercial tea beverages contain such small amounts of polyphenols that you would have to drink 20 bottles to get an amount equal to what's in one cup of home-brewed tea.

Polyphenols are believed by scientists to have anti-cancer, anti-inflammatory and anti-diabetic properties.

"Consumers understand very well the concept of the health benefits from drinking tea or consuming other tea products," researcher Shiming Li said in an American Chemical Society news release. "However, there is a huge gap between the perception that tea consumption is healthy and the actual amount of the healthful nutrients -- polyphenols -- found in bottled tea beverages. Our analysis of tea beverages found that the polyphenol content is extremely low."

Li and colleagues measured the levels of polyphenols in six brands of bottled tea beverages purchased at supermarkets. Half of them contained virtually no antioxidants, while the polyphenol levels in others were so low that they would have little effect on health.

"Someone would have to drink bottle after bottle of these teas in some cases to receive health benefits. I was surprised at the low polyphenol content. I didn't expect it to be at such a low level," said Li, an analytical and natural product chemist at New Jersey-based biotechnology company WellGen Inc.